1) Field of the Invention
The present invention relates to airflow improvements in coal pulverizers, and more particularly, to a system for improving performance on a vertical spindle coal pulverizer by providing a secondary airflow path introduced above the grinding surface that is directed downward for sweeping across the grinding zone.
2) Description of Related Art
Pulverized coal fired power plants produce electricity and/or steam for industrial use through the use of a boiler which converts water to steam that is then either utilized for industrial use or to power a turbine which powers an electric generator. Raw coal is feed to the pulverizer in chunks which are then ground within the pulverizer, dried and transported with air to the burners connected to the boiler. Pulverizer performance is critical to the performance of the boiler.
One specific type of pulverizer is known as a Vertical Spindle Coal Pulverizer. These types of pulverizers operate by having the raw coal chunks feed onto a rotating grinding surface. The coal is then forced between grinding elements, typically large journals/tires. A primary airflow is feed upward through the pulverizer from a lower housing portion around the grinding surface, which acts as the transport for the coal “fines” or pulverized particles of coal to move upward to the classifier. This primary airflow also dries the coal. The area around the rotating grinding table is referred to as the pulverizer throat. Raw coal also has pyrites which are rejected or allowed to pass through the pulverizer throat and exit the pulverizer under the grinding surface. In order to achieve desired pulverizer and burner performance the ratio of fuel/coal to air is critical and must be measured and controlled.
Pulverizer performance includes but is not limited to the particle size distribution, air/fuel ratio, air and fuel distribution leaving the pulverizer, amount of coal being rejected out of the bottom of the pulverizer and drying ability of the mill in order to maintain controllable pulverizer outlet temperatures. A major factor in performance that impacts all of the above is how the air enters the pulverizer at the pulverizer throat located around and generally below the grinding surface. High pulverizer differential pressure between a lower housing and upper housing is a common issue which can limit primary airflow capacity and thus pulverizer throughput/capacity. Mill rumble due to a bed of coal fines building up on the table and causing the journals/tires to skid is also a common issue resulting from airflow limitations. Also, coals with high levels of silica can result in the accumulation of sand beds which are high density and difficult if not impossible to remove with current throat configuration and high differential pressures.
Airflow of approximately 7,000 fpm velocities (terminal velocity of raw coal) must be maintained in order to prevent coal from being rejected through the pulverizer throat while still allowing larger pyrites to be rejected through the throat. A problem with most typical pulverizer throats is that they are not sized properly and thus do not reach the necessary airflow velocity at low load/coal throughput. Attempts to correct this type of problem result in larger amounts of airflow being introduced into the pulverizer, which result in increased differential pressures and high air to fuel ratios that adversely affect pulverizer and burner performance (poor fineness, poor distribution out of the pulverizer, higher than desired velocities at the burner impacting combustion).
Further, if coal is allowed to spill to the lower housing portion or underbowl, which is the same area in which the primary air enters, it gets heated to the primary air temperature and if it is not removed can result in mill fires. Accordingly, a need has arisen in maintaining a proper airflow and reducing differential pressures in the pulverizer to move coal fines upward through the pulverizer to the burners will allowing other materials to pass downward through the throat.
Accordingly, it is an object of the present invention to provide a secondary airflow into the mill housing from above the grinding surface bypassing the pulverizer throat to improve airflow dynamic and pulverizer efficiency.
It is another object of the invention to maintain lower pulverizer differential pressure by introducing a secondary airflow into the mill housing above the pulverizer throat and grinding surface.
It is another object of the invention to reduce low load mill rumble by clearing fine particles from the grinding zone by directing a secondary airflow entering the upper housing downward across the grinding surface.
It is another object of the invention to control a secondary airflow into the upper housing of the mill above the grinding surface to adjust air/fuel ratios according to load.
It is another object of the invention to improve pulverizer performance by providing primary classification at the grinding zone by efficiently removing the coal fines and allowing non-coal materials to be rejected through the throat.